1. Field of the Invention
This invention relates to a blowing head for use in blown cable installations and to a buckle detector suitable for integration with such a blowing head.
2. Related Art
It is well known from European Patent EP-B-108590 and subsequent publications that optical fibre transmission lines can be installed into ducts using the fluid drag provided by a high-speed flow of a gaseous medium. This method is practiced with optical fibre and copper cables ranging in size from about 1 mm to 20 mm or more, depending upon the application and the route into which installation is to take place.
As described in EP-B-108590 a blowing head comprises a pair of motor driven wheels which provide a propulsive force which serves to advance the cable until a sufficient length has been installed in the duct for viscous drag forces to be effective in further installing the cable (typically the first third to half of the overall length of the installation duct). The drive wheels also enable the hydrostatic back pressure, which acts to urge the cable back out of the duct, to be overcome.
The rate of the advance of the cable depends inter alia upon the magnitude of the pushing force applied by the drive wheels, friction and the amount of viscous drag available (which depends upon the applied gas pressure, cable diameter, duct size and length and installed cable length). Typically the rate of installation in any given situation is controlled by the operator simply controlling the amount of pushing force applied by the motor. Because any given blowing head can typically be used with a range of cable diameters, it is common for the blowing head motor to be able to provide more pushing force than a relatively small cable can withstand without buckling. Normally the desire is to use the fastest possible installation rate while avoiding buckling and the motor control is adjusted to provide this. As those skilled in the art will understand, in practice a given cable will start to buckle under different conditions depending upon its free length, the pulling force provided by viscous drag, friction, duct diameter, duct geometry, etc. Thus, in practice the machine operator needs to stay with the blowing head in order to monitor the blowing process and to ensure that buckling is avoided, at least if a reasonable installation rate is to be achieved.
Cable buckling is avoided in order to prevent damage to both the cable and blowing head, especially the drive wheels. Buckling can cause the cable to jam in the duct and tends to cause delays in the installation process. Blowing heads may include motor stall sensors, clutches or frangible links operative to prevent damage to the motor or drive wheels (or tires). Even with heads having such features, the operator in practice still needs to monitor the blowing operation at the blowing head. A device, known as a buckle detector, for monitoring the onset of buckling and for controlling the blowing head drive to avoid buckling is described in EP-B-253636. It is known from GB-A-2 294 131 to integrate a buckle detector with a blowing head.
The present applicants, who are the proprietors of the above-mentioned European Patents, have now devised a new buckle detector which can be integrated with a blowing head to form a simple and compact device which improves overall blowing performancexe2x80x94even compared to the combined use of the blowing head of EP-B-108590 and the buckle detector of EP-A-345968. The new head design resulting from the integration of the new buckle detector is particularly suitable for use with the very compact optical fibre cables described in EP-A-521710 and EP-A-345968.
According to a first aspect of the invention there is provided a blowing head for a blown cable installation process, the blowing head comprising:
a cable inlet to receive a cable, a cable outlet for connection to a duct into which the cable will be installed, the cable inlet being connected to the cable outlet by a connecting channel,
one or more drive wheels coupled to drive means, the drive means, in use, driving the one or more drive wheels to apply a longitudinal drive force to the cable,
a cavity contiguous with the connecting channel, deflecting means, sensing means, control means, a gas inlet port and a gas injection point, the gas injection point being coupled to the gas inlet port and opening into the connecting channel at a point intermediate the cavity and the cable outlet;
said longitudinal drive force in use causing the cable to be advanced along the connecting channel, through the cavity, past the deflecting means and the gas injection point, and through the cable outlet into a duct connected thereto;
There is also provided a blown cable installation apparatus, comprising a blowing head as described above and control means responsive to the output of a sensor in the buckle detector and operative to control the motor in response to the output of the sensor.
According to a second aspect of the invention there is provided a blowing head for a cable installation process, the blowing head comprising: a cable inlet to receive a cable, a cable outlet for connection to a duct into which the cable will be installed, the cable inlet being connected to the cable outlet by a connecting channel, one or more drive wheels coupled to drive means which, in use, drives the one or more drive wheels to apply a longitudinal drive force to the cable, a cavity contiguous with the connecting channel, deflecting means, sensing means, control means, a gas inlet port and a gas injection point, the gas injection point being coupled to the gas inlet port and opening into the connecting channel at a point intermediate the cavity and the cable outlet; the longitudinal drive force causing the cable to be advanced along the connecting channel, through the cavity, the deflecting means and the gas injection point, and through the cable outlet into a duct connected thereto; the sensing means being configured to sense lateral deviation of the cable, said lateral deviation occurring when excessive longitudinal drive force causes the cable to buckle at the deflecting means; the control means being responsive to the sensing means to control the drive means to vary the driving force applied by the one or more drive wheels in accordance with the output of the sensing means. Preferably means are provided to detect the direction of movement of the transmission line such that if the transmission line moves in a reverse direction (from the usual installation direction) the control means direct the drive means to increase the driving force applied by the one or more drive wheels until the transmission line is held in equilibrium. Preferably only one of the drive wheels is coupled to the drive means. Preferably the sensing means comprise an optical source and an optical detector.
Preferably the source of high pressure gas is chosen to be compatible with the environment in which the installation will take place, although routinely an inert gas such as air or nitrogen will be used. A compressor or bottled gas may be used.
According to a further aspect of the invention there is provided a method of installing a cable into a duct, the method comprising the steps of:
i) coupling a blowing head according to the first or second aspects of the invention to the duct;
ii) introducing a cable into the blowing head and engaging the cable with the rotatable driving element or wheel of the head;
iii) applying pressurized gas to the duct;
iv) advancing the cable through the head and along the duct; and
v) controlling the tractive force applied through the rotatable driving element or wheel in accordance with the output of a sensor or sensors of the blowing head.
According to a further aspect, the present invention provides a blowing head for a blown cable installation process, comprising: a motor and a rotatable driving element operatively connected to the motor; a cable inlet to receive a cable to be installed; a cable outlet for connection to an installation duct into which the cable is to be installed, the cable outlet being connected to the cable inlet by a through bore, the through bore defining a cable path along which the cable passes in use; a rotation sensor to sense the direction of movement of the rotatable driving element; and control means, operatively connected to the rotation sensor and to the motor. In use, the control means causes, in response to the detection of rotation of the driving means in the sense opposite to that which applies when the cable is being inserted into the duct, the motor to apply a holding torque to the driving element to counteract the opposite rotation.
In a yet further aspect, the present invention provides a buckle detector for a blown cable installation process for use downstream of the tractor element of a blowing head, the detector comprising: a cable inlet to receive a cable to be installed; a cable outlet for connection to an installation duct into which the cable is to be installed, the cable outlet being connected to the cable inlet by a through bore, the through bore defining a cable path along which the cable passes in use; a cavity, through which the cable path passes, a sensor or sensors being provided to monitor, in use, the position of the cable within the cavity. The buckle detector also includes a deflector on which the cable impinges on its passage along the cable path, the deflector being arranged to introduce a bend into the path followed by the cable. If the cable is propelled into the buckle detector with an excess of force the cable will preferentially buckle within the cavity at the site of the deflector.